Fibre Lasers

High-power fibre lasers (greater than 1 kW) suitable for industrial applications in metals processing are a recent entrant to the field of available processes (Rossi, 2000).

Fibre lasers offer potential for significantly cheaper ownership costs compared to the case for even recent entrants to the field of commercial lasers. The floor space footprint of such lasers is considerably smaller than that of an equivalent powered Nd:YAG laser and the laser ‘wall plug’ efficiency of 20% is considerably higher than the 3% typical of Nd:YAG lasers, resulting in a significant reduction in the

requirements for chiller capacity and hence size also. .

Yb fiber-laser technology (1.07-m wavelength) has been incorporated into the hybrid welding technique, and with the higher electrical efficiency and smaller footprint of these lasers threatens to replace CO2(10.6-m wavelength) and Nd:YAG (1.06-m wavelength) lasers where flexible automation and higher laser power (above 5 kW) are considered. This emerging laser technology produces equivalent or

superior beam quality to current lasers, but has reduced infrastructure requirements for installations (i.e., cooling water, power, floor space), which simplifies

implementation into typical production environments.

Work in Europe (Herbert, 2004) reports use of high-power fibre lasers up to 10 kW. The emission wavelength ranges from 1.07 to 2.0 µm so performance equivalence in terms of fibre-optic beam delivery and its use compared to an Nd:YAG laser is comparable for LMW/GMAW hybrid work.

The latest commercially available fibre lasers are at the point where portability and field use of such systems is being seriously considered. One example is for onshore and offshore pipeline construction, and research is presently being conducted in Europe and at EWI for cross-country pipeline applications, enabled particularly by the small size and rugged portability of the high-power fibre lasers, up to 10 kW,

available today.

2.10 Weld Bead Humping

With fusion welding including arc welding and laser welding, as the welding TS increases, the progression is as follows:

 Smooth beads  Uneven beads  Undercut

 Continuous bead humping (and undercut)  Discontinuous bead humping

Humping refers to the formation of an irregular weld bead profile characterized by repeating peaks and valleys in the weld metal, and is a defect found in fusion welding processes as the welding speed approaches a limiting value.

The appearance, at successively higher TS, of first continuous and then discontinuous bead humping is illustrated in Figures 2.18 and 2.19, respectively.

Figure 2.18 Classic Continuous Bead Humping in GMAW-P BOP

In the former case, Figure 2.18, the weld bead, albeit narrower, is continuous between one hump and the next, while in the latter the arc gouged profile can be seen between separated humps of weld metal, Figure 2.19. The welds in Figure 2.18 were made at 2 m/min with 10 m/min WFS, while that in Figure 2.19 was made at 2.5 m/min TS with 10 m/min WFS.

Figure 2.19 Weld Appearance for Classic Discontinuous Bead Humping in a GMAW-P Lap Joint (Paskell, 1989)

The examination of the humping mechanism and its suppression to achieve higher welding speeds for increased productivity is needed to determine the fundamental cause and mechanisms and thus to address a remedy for bead humping. Use of high-

speed video (HSV) is needed for direct observation of metal transfer and weld pool behavior for GMAW and observation of the hybrid LBW/GMAW processes.

No work has been done, and is therefore needed, in the area of laser beam splitting to examine its effect on weld toe wetting of both weld toes simultaneously as a method to overcome weld bead humping for sheet metal applications.

Weld bead humping occurs both in GMAW and LBW, at TS related to maximum weld bead width to length ratios. The observed sequence of events in GMAW is as follows:

1. Circular weld pool 2. Elliptical weld pool

3. Teardrop-shaped weld pool 4. Continuously humping weld pool

5. Discontinuous humping of the weld pool.

The weld pool shape is elongated as TS increases and begins to hump as the weld toe contact angle is reduced below 90 degrees. This has been studied in work on weld bead sizing (Boring and Harris, 2004) and weld bead humping (Harris et al., 2005), from the context of solutions through welding process research and development.

In LBW, humping occurs when TS relative to welding power and weld pool size follows the following sequence (Albright, 1988):

1. Keyhole mode 2. Conduction mode

3. Humping bead in conduction mode

The last mode is analogous to the humping mode in arc welding.

2.11 Weld Pool Dynamics and Humping Mechanisms

The phenomenon of discontinuous bead humping in GMAW was first systematically studied and documented in 1968 (Bradstreet, 1968). Bradstreet used a simple but effective precursor technique similar to weld pool decanting to study weld pool shape and the bead humping phenomenon, whereby the plate was hinged to allow the weld pool to drop out under gravity. His findings from visual observation of weld pool fluid flow are summarized in Figure 2.20, for a weld pool of about 25-mm length. This shows the predominant weld metal fluid flow to be rearward, and also that the unsolidified weld metal, once it reached the rear of the weld pool at the solidification front, travelled back toward the front of the pool.

Figure 2.20 Weld Pool Fluid Flow as Observed and Proposed by Bradstreet for High-Speed GMAW (Bradstreet, 1968)

A study of the effect of arc pressue on defect formation in GTA welding (Savage et al., 1979) found higher TS limits before the onset of humping, roughly 1 m/min compared to 0.5 m/min could be achieved by using helium shielding gas rather than Ar shielding. Equipment developed to measure arc force directly found no difference in the arc force measured in the two cases. The arc force varied from 3 g at 100 A up to 15 g at 550 A. They concluded that arc pressure was the dominant mechanism at welding current above 250 A, and surface tension was the cause of defects at welding current below 250 A. This implies that, at least for GTAW, the arc force at higher current can overcome the effect of surface tension as the two compete in terms of weld shape and fluid flow. However, the pool dynamics are considerably more

complicated when using GMAW, since the effect of droplet impingement on the weld pool surface is also involved.

The effect of minor amounts of sulphur on weld penetration through reversal of Marangoni fluid flow in stainless steels is well known (Heiple and Roper, 1982). This affect is also caused in steels as well as stainless steels, but is certainly best known and characterized in stainless steels.

Summary work on the phenomena and measurement of weld pool surface tension (Rodwell, 1985) discussed the effect of oxygen content of the weld pool on lowering surface tension and improving weld pool wetting at the bead toes. Weld bead

humping and undercutting are normally caused by excessive welding speed that causes the weld metal to solidify before surface depressions created by the arc forces can be completely filled. Surface tension forces control the degree of wetting at the edges of the weld pool, which is improved if oxides are present on the molten weld pool. A low weld pool surface tension promotes good wetting and thus increases the opportunity for the molten metal to refill the groove before solidification occurs. Increasing the oxygen content of the shielding gas from 1 to 5% had an appreciable effect in increasing toe wetting. Since typical shielding gases for GMAW of steel contain CO2rather than oxygen, it is worth noting that the oxidizing potential of

oxygen is 2.5 times greater than that of CO2. Thus a 5% oxygen content has the same oxidizing potential as 12.5% CO2.

Modeling of phenomena associated with partial wetting (Sekimoto et al., 1987) describes the movement of contact lines at the edges of liquids wet to the surface of a solid. While this has much more in common with wetting of brazing alloys than weld pools, it offers some insight into local instabilities, which have parallels in the weld bead humping mechanism in terms of contact lines at the weld toes, which are not straight, but neck down in the transverse axis between humps.

Significant work on weld profile defects in laser welding (Albright and Chiang, 1988) for sheet steels characterized a number of defect types from cutting and hole

formation to undercut, uneven bead, and humping defects. Thin material of 0.13 and 0.25 mm were used along with high welding speeds in the range of 1 to 5 m/min. Laser powers of 450 W to 3.3 kW were used employing a transverse flow CO2laser. All welding was conducted in a BOP mode. The results were plotted in discontinuity maps that showed boundary conditions for the various defect types along with regions where smooth (defect free) welds could be produced. As with arc welding processes, smooth beads became uneven, undercut, and then of humped (and undercut)

appearance at successively higher TS.

A review (Paskell, 1989) of weld bead humping in GMAW showed that work in the 20 years since that of Bradstreet had not significantly advanced the field in terms of understanding or a solution.

Marangoni convection was proposed as the underlying mechanism for humping (Mills and Keene, 1990) but later work (Gratzke et al., 1992), showed that surface tension, acting in a manner similar to that breaking up a liquid cylinder was an important underlying physical mechanism, and that the length to width ratio of the weld pool was more important, the ratio needing to be less than 10:1 for arc welding to prevent humping.

Heat conduction in finite thickness, i.e., applicable to welding of sheet thicknesses for laser welding was studied (Gratzke et al., 1991) for steel to calculate the fusion boundary position. High-speed laser welding was studied by Albright and Chiang (1988) and characterized a number of welding defects such as humping, undercut, ropy (i.e., uneven) bead shape, cutting and hole formation. For conduction mode welding the molten weld pool increases in length with welding speed and power as is the case in arc welding. They approximated the weld pool to a liquid cylinder as noted in other work involving Gratzke.

Studies on humping effects (Gao and Sonin, 1994) (Schiaffino and Sonin, 1997) showed that the humping instability depends on the conditions at the boundary of the deposit, i.e., the weld toe region, in the case of a weld bead produced by GMAW. They argued, similarly to Gratzke, that the mechanism involved in bead hump formation is the surface tension of the molten metal surface. As TS is increased, the time required for conduction cooling and solidification of the weld deposit is

somewhat decreased, but not in proportion to the TS increase. This leads to a molten weld pool with a relatively large length/width ratio, a fluid geometry that is prone to a dynamic shape instability controlled by surface tension. This instability is similar to

the familiar one that causes a thin stream of water falling from a tap to break into droplets. In the case of welding, if the surface tension forces within the weld pool are sufficiently strong, then the bead can develop appreciable humps before cooling freezes the deposit geometry. The humps form as a result of the transverse movement of the molten metal as it tends to neck down laterally driven by surface tension to lower the overall surface energy.

Schiaffino and Sonin in 1997 (2) showed that if the melt and target material (weld pool and base material) are the same then solidification always takes place near a moving contact line, or weld toe. From visual observation, welding forms humps based on a moving contact line as in sketch (b), Figure 2.21, rather than in sketches (a) or (c). This makes sense since naturally a weld solidifies from the fusion boundary into the melt, and the observation takes some account for the shrinkage feature often seen in the surface of the peak of a humped weld bead, based on solidification shrinkage occurring there as it is the last part of the bead to solidify.

Figure 2.21 Contact Line Behaviour [Schiaffino and Sonin, 1997 (2)]

The Marangoni effects of sulphur in a GTAW pool are well known and have been characterized by many over the years. More recent work (Mills et al., 1998) also characterized the effect of oxygen in terms of slag spots and oxide films. The soluble oxygen is considerably lower than the total oxygen since oxygen readily forms Al2O3 in the presence of Al additions in excess of 20 ppm. The soluble sulphur content on the other hand is similar to the total sulphur level unless the steel is Ca or Ce treated.

The combined oxygen has little effect on the surface tension. The situation for GMAW is more complicated since there are more forces at work with the addition of molten droplets from the electrode wire, and as the wire has a sulphur content of its own which may well be different to that of the base material.

The observations of other research (Schiaffino and Sonin,1997 (2)) were interesting in that they noted that Gratzke et al. (1992) rejected the thermocapillary mechanism and proposed that humping was caused by Rayleigh instability, i.e., the breakup of a liquid cylinder by the action of surface and gravity forces. Gratzke concluded that (i), the width/length ratio of the weld pool was the most important factor ,and that (ii) the surface tension does not affect the onset of humping, only the kinetic behaviour. Mills et al. observed that this latter conclusion seems to be inconsistent with the observation that humping is prevalent in high sulphur casts of base metal. Based on the competing findings and opinions of Schiaffino and Sonin, Gratzke, and Mills et al, is seems most likely that surface tension is indeed one of the dominant forces

involved in the mechanism of humping. Gratzke’s proposed Rayleigh instability theory has some merit, but mostly as it involves the effect of surface tension, rather than gravity.

Work on weld pool dynamics and weld pool fluid flow in the 1980s concentrated mostly on the GTAW process (Kujanpaa, 1983), (Oreper and Szekely, 1984), (Lin and Eagar, 1985), lower welding currents, and significantly lower TS than typical of GMAW. Recent work (Mendez and Eager, 2003) dealt with higher current GTAW. However, the GTAW process, without the addition of filler metal, is more amenable to analysis than the considerably more dynamic GMAW process. Theoretical work on GMAW (Ushio and Wu, 1997) and contact tip to workpiece effects in GMAW (Kim and Na, 1995) dealt with welding currents up to 250 A, and the latter showed the results of modeling on the weld pool fluid flow, Figure 2.22. This is consistent with the observations of Bradstreet (Bradstreet, 1968). However, no observations were made on the velocity of motion of the weld pool. Work is needed in this area to generate data for the speed of weld metal fluid flow in the weld pool. Such work could be conducted using HSV.

Figure 2.22 Rearward Weld Metal Fluid Flow Including the Effect of Arc Pressure and Wire Droplet Addition (Kim and Na, 1995)

Independent research was conducted and reported (Nguyen et al., 2005) within the timeframe of the author’s research. The study is the first one since Bradstreet to address the phenomenon of humping in GMAW in a process-specific and systematic method. The work used LaserStrobe equipment with nitrogen laser backlighting. Video image capture was used to examine the formation of the humps and they concluded that the humping defect was caused essentially by the dominant rearward motion of the weld metal flow. The rearward motion was established by earlier researchers, but does not explain the mechanism by which one hump ends and the next begins. Evaluation and determination of this mechanism is therefore needed. Images presented in the paper show the welding torch was used at a perpendicular angle to the workpiece and weld pool. Suggestions in the conclusions of the paper were made to incline the workpiece 10-degrees downhill to use gravity assistance to overcome the rearward momentum of the weld pool and achieve higher welding speeds. This is considered an impractical suggestion for most applications. However, the use of a leading torch travel angle to alter the process and metal transfer dynamics was absent from their work and needs to be studied to increase the TS for the onset of weld bead humping. A very significant practical factor was that all the welds were made in bead on plate mode, with no evaluation on welded joints. They evaluated Transferred Ionized Molten Energy (TIME) gas and achieved high welding speeds up to 3 m/min before humping. They also used a benchmark of Ar shielding gas which is not a typical gas used for GMAW of steel. The differences in TS for the onset of humping between the Ar-CO2shielding gas they used and the TIME gas were not significant for most arc powers, only being higher at 5-kW arc power – 3.6 m/min for the TIME gas versus 2.5 m/min for Ar-CO2. The ability to minimize humping with a larger weld pool depth has to be balanced with the need to avoid excessive penetration and burnthrough when welding sheet products and is a limiting condition or boundary condition for the upper TS for humping suppression.

Another conclusion of their work was that the metal transfer, known technically as rotational spray transfer, occurred at arc power above 7.5 kW for the 0.9-mm diameter electrode wire, reduced the vertical component of the arc force and caused less arc gouging depth, thus increasing the TS at which humping occurred.

Weld pool decanting was used in a weld pool modeling study (Wahab and Painter, 1997) to determine weld pool shape and the effect of higher welding current and TS on the length and metal flow in the weld pool. Higher currents produce higher arc force and droplet impact on the weld pool. This creates a rearward flow of molten weld metal to the rear of the pool. Thus, as current increases and TS increases, the weld pool lengthens. They determined experimentally through weld pool decanting that the development of a tail on the weld pool is directly linked to welding current and TS.

The possibility of combining LBW with GMA (MIG/MAG) welding was studied, (Choi, Farson, and Cho, 2006) to enable high-speed welding whilst avoiding hump formation in the weld bead. BOP welds were prepared on hot-rolled 1008 mild steel sheet of 1.6- and 2.0-mm thickness. GMA welds were produced using a pulsed power source, ER70S-5 filler wire, contact tip-to-workpiece distance of 22 mm, and Ar or Ar-10%CO2shielding gas. In the welds made using the hybrid process, the laser spot (2.0- to 3.5-kW power, 3- to 5-mm diameter) preceded the arc by 3-5 mm. TS, laser power, and laser spot width were varied. Weld bead shape was characterised by

measurements of height and weld toe angles. Weld bead height was compared for welds made by the GMA and hybrid processes. This recently published work used in- line GMAW and laser heat sources to alter the heat input, weld pool shape, and weld pool fluid flow. These researchers found that the humping defect could be reduced at 2 m/min travel speed and concluded that this was associated with increased heating and weld toe wetting, not only capillary instability. The work was a precursor to research conducted as part of the experimental approach in this thesis, where offset laser beam spots were used to impinge the weld toes rather than the centerline of the